A fluid dynamic bearing device is configured to relatively rotatably support a shaft member in a non-contact manner by a pressure (dynamic pressure generating action) generated by a fluid film filled in a radial bearing gap defined between an outer peripheral surface of the shaft member and an inner peripheral surface of a bearing member. The fluid dynamic bearing device has advantages in high rotational accuracy and quietness. Thus, the fluid dynamic bearing device is preferably usable for a spindle motor for information equipment (for example, magnetic disk drives such as an HDD, drives for optical discs such as a CD-ROM, a CD-R/RW, a DVD-ROM/RAM, and a Blu-ray Disc, and drives for magneto-optical disks such as an MD and an MO), a polygon scanner motor for a laser beam printer (LBP), a color wheel for a projector, and a small-sized motor such as a fan motor to be used for a cooling fan or the like of an electrical apparatus.
As the bearing member to be used for such a fluid dynamic bearing device, a sintered bearing made of sintered metal may be used. The sintered bearing can be formed into a near net shape. Thus, finishing can be reduced or omitted, and hence the sintered bearing can be manufactured at low cost. Further, when dynamic pressure generating grooves are formed in an inner peripheral surface of the sintered bearing, the dynamic pressure generating grooves can be die-formed in the inner peripheral surface of the sintered bearing in a sizing step. Accordingly, formation of the dynamic pressure generating grooves is facilitated as compared to a case of forming the dynamic pressure generating grooves by etching or the like.
In Patent Literature 1 listed below, there is described a specific method of die-forming a radial dynamic pressure generating portion (dynamic pressure generating grooves) in the inner peripheral surface of the sintered bearing. In this method, first, metal powder is subjected to compression molding and then sintered, thereby forming a sintered body (sintered metal material). Then, a core rod having a forming pattern formed in an outer peripheral surface thereof is inserted along an inner periphery of the sintered body, and the sintered body is press-fitted to an inner periphery of a die while the core rod remains inserted along the inner periphery of the sintered body. In this manner, the sintered body is compressed from an outer periphery thereof, and an inner peripheral surface of the sintered body is pressed onto the forming pattern of the core rod. Consequently, the inner peripheral surface of the sintered body is plastically deformed, thereby die-forming the dynamic pressure generating grooves. After that, while the core rod remains inserted along the inner periphery of the sintered body, the sintered body is taken out from the inner periphery of the die. At this time, the sintered body is released from a compressive force that is inwardly applied to the sintered body, and spring back occurs in the sintered body so that the inner peripheral surface of the sintered body is increased in diameter. As a result, the inner peripheral surface of the sintered body is separated from the core rod. Thus, the core rod can be pulled out from the inner periphery of the sintered body without causing interference between the dynamic pressure generating grooves formed in the inner peripheral surface of the sintered body, and the forming pattern of the core rod.